1. Field of the Invention
The present invention relates to a coking drum according to the preamble of claim 1 and to a coking method, which is designed especially for coking residues from the vacuum distillation of crude oil, according to the preamble of claim 13.
2. Description of Related Art
In crude oil processing, value is increasingly being placed on the further processing of heavy products left over from the distillation of crude oil to lighter products. Accordingly the production of heavy heating oil is to be reduced and the production of gasoline, diesel fuel and light heating oil increased without the need to process additional crude oil. The plants required for that purpose, which operate according to various methods, are called conversion plants. They convert heavy, long hydrocarbon molecules into light, shorter hydrocarbon molecules by cracking the long molecules. A distinction is made between three such cracking methods: thermal cracking, catalytic cracking and hydrocracking. The optimum combination of methods depends on several factors which include, inter alia, the quality of the crude oil in question and the products desired. The various cracking methods are based on different feed products, gas oil from vacuum distillation acting as the feed product for catalytic cracking units and hydrocrackers, and the residue from vacuum distillation being used as feed product in visbreakers or cokers.
The longest known and simplest cracking method is thermal cracking. In that method hydrocarbon chains are cracked at high temperatures. The group of thermal cracking methods includes visbreaking and coking in which carbon in solid form, so-called coke or petroleum coke, is deposited.
There are three different methods of coking, namely fluid coking, delayed coking and flexicoking. The most frequently used method is delayed coking. In that method the feed product which, for example, may be the residue from vacuum distillation, is introduced into a furnace at a pressure of about 30 bar and heated to about 500° C. As a result of those conditions, the feed product flows through the furnace at very high speed and then cokes when admitted into a coking chamber or coking drum having a prevailing pressure of about 4 bar that is separate from the furnace and connected thereto by a pipeline.
There are generally associated with each furnace at least two coking drums, one of those chambers in each furnace being in operation, while coke is being removed from the other. The coke can be cut out of the coking drum by means of, for example, water under elevated pressure. The light hydrocarbons produced during the coking process are conveyed from the coking chamber into a fractionating tower where they are further processed. The resulting petroleum coke is called green coke and, after being comminuted, can either be sold or further refined. Further refining takes place in a calcining process in which, at temperatures of 1200° C. and above, any oil constituents still present are burnt off and coked. The calcination product obtained in that process can then be used, for example, for electrodes, which are employed in the aluminium industry.
The drum that has been filled is cooled, and the coke is removed. For that purpose water is first of all introduced into the drum in order to cool the hot coke obtained. The drum is then opened to the atmosphere by, depending on the design of the coking chamber or drum, either only the bottom end of the drum or chamber being opened or also the top end of the drum or chamber being opened, so that the coke can be cut out of the drum and delivered for further use.
The operation of opening the coking drum may present sources of risk for several reasons. The water introduced into the drum for cooling the coke prior to the drum being opened is very hot, and, if the equipment is not handled carefully, may result in injuries caused by the emerging hot water or steam, which is under pressure. In addition, loose bits of coke may fall out of the drum or place such a strain on the opening mechanisms that, after they have been unlocked, open abruptly, which may also result injuries to the operating personnel. The operating personnel may also be exposed to dust, especially coke particles and also irritant or noxious gases, when the drums are opened.
To reduce the sources of risk to operating personnel, a change was made from opening the coking drums manually to opening them automatically by means of dedicated, for example hydraulic, operation of the lid. Such a coking drum having an automatic opening mechanism is known from WO 02/072729 A1. The coking drum-opening device illustrated in that specification is an automatically operated slide valve which, in the open position, allows entry to the coking drum whilst, in the closed position, the slide plate seals the coking drum from the atmosphere.
A disadvantage of the device mentioned is the moderate reliability in respect of unintentional escape of hot water or steam and gases from the coking drum. In addition, malfunctions structurally may occur, since coke or the like bakes onto the guide rails of the slide valve, especially on the side of the guide rails facing the coking chamber.